It is a commonplace in shipyards, foundries, manufacturing plants and alike, to hoist and convey heavy articles suspended from an overhead traveling crane by an assembly of hooks and chains commonly called "slings" by the art. As a consequence of misuse, overloading or improper loading, and jerky crane operation among other factors, elements of the sling become visably worn, gouged or deformed and in some situations, stressed to provide cracks not readily apparent by casual visual inspection.
Federal and State safety regulations require annual testing of the sling components by certified inspectors to insure a safety factor of at least 2:1, i.e., that a chain having a 1,000 pound rating, for example, can withstand a load of at least 2,000 pounds. Such testing requires special heavy and sophisticated equipment not available at the industrial site where the sling is used. Heretofore, private testing laboratories having such equipment have collected slings from the industrial sites for annual testing, transported the slings to a test facility, tested the sling components and if necessary and feasible, repaired the same. After being tested, the sling components are then certified as having the required 2:1 safety factor and returned to the industrial site where the sling is to be used. The annual certification usually entails a day or more downtime for the sling before it is returned to the user, who is thus loath to dispense with the sling more frequently than absolutely necessary to abide by the law, regardless that more frequent thorough testing is often advisable for slings in constant demanding use.
Cleaning and visual inspection at frequent intervals, depending upon the extent and severity of use, is recommended by the sling manufacturer, and in fact, where slings are in constant severe use, daily cleaning and inspection is recommended. In order to accommodate such recommendations, certified inspectors travel to the industrial site where the slings are used, then clean and visually inspect the same link by link for obvious wear and defects. Although an expert inspector can often detect visable wear and deformation of the sling components, such as a chain link that has been elongated by prolonged use, or a hook that has been weakened by tip loading, such inspections offer a minimum of protection and are not adequate for safety. Elements of the sling can become dangerously weakened by fatigue or minute cracks invisible to the unaided eye, and detectable only by more sophisticated procedures. Thus, in addition to frequent visual inspection, sling components in constant heavy use should be stress tested or examined under ultra-violet light by conventional magnetic flux procedure at intervals throughout the year to assure that they may be safely used up to their rated capacity.
Obviously where sling components sustain damage or fatigue that is not detectable by visual inspection, the potential exists for sling failure under a high tonage load and serious injury to nearby personnel.
In view of the reluctance in some instances of the owners and users of industrial slings to send their slings to an inspection and repair facility, the need for practical facilities to enable frequent comprehensive inspection of industrial slings above and beyond visual inspection at the industrial site where the sling is used has existed for many years, at least since the introduction of the overhead crane in heavy industry.